<p>Marine fish markets are important for urban nutrition in Bangladesh. However, systematic studies on how the urban market structure affects fish availability, pricing, demand, and affordability are lacking. This study evaluated the market structure, price formation, consumer demand behavior, and affordability of marine fish in Chattogram, Bangladesh’s largest coastal city. Data on 42 marine fish species from 20 different retail markets were analyzed via diversity indices, nonmetric multidimensional scaling analysis, price formation models, instrumental variable two-stage least squares (2SLS), and a daily affordability index (DAI). Species diversity was consistently high across markets, whereas multivariate analyses identified three distinct market clusters. Retail availability differed significantly from national production patterns, with pomfret, tuna, and jewfish being overrepresented and hilsa, Bombay duck, and sardines being underrepresented. Elasticity analysis revealed that aggregate demand for marine fish in Chattogram is relatively inelastic but remains responsive to price changes. A simulated 10% supply reduction increased projected retail prices by 8.10%. Affordability analysis indicated that 95.2% of species were either highly or moderately affordable, with no species classified as unaffordable. The findings suggest that market competition and supply-chain dynamics influence fish accessibility and consumer welfare. Improving market infrastructure, competition, and distribution efficiency may enhance affordability, strengthen food security, and increase the resilience of urban fish markets in Bangladesh.</p>